The present invention relates to a power tool including a motor serving as a drive source for a bit.
Japanese Laid-Open Patent Publication Nos. 2010-214520 and 60-207776 each disclose a power tool that drives a bit with a motor serving as a drive source. Japanese Laid-Open Patent Publication No. 2010-214520 (paragraphs 0020, 0033, and 0054, and FIGS. 1 and 2) describes a power tool including a motor, a gear mechanism, switching elements of a motor drive circuit, and a temperature sensor. The gear mechanism shifts the speed of the rotation output from the motor and transmits the rotation output to a rotation output unit to which a bit is attached. The temperature sensor measures the temperature of the switching elements. When the temperature of the switching element measured by the temperature sensor becomes higher than a set temperature, the power tool restricts or stops the rotation produced by the motor. The motor includes an output shaft. A fan is coupled to the output shaft. In the motor, the fan is rotated to produce an air current that cools the switching elements. The switching elements are fixed to the same heat radiation member and mounted on a circuit substrate. Further, the switching elements and the circuit substrate are arranged on opposite sides of the motor.
Japanese Laid-Open Patent Publication No. 60-207776 (left column in page 1 (claim 2), lower left column in page 4, FIGS. 9 and 11) describes a power tool that measures the temperature of a switching element (triac) with a temperature sensor. The power tool controls the rotation speed of the motor and stops the motor based on the measured temperature. A bracket is used to couple the switching element to a stator of the motor. The temperature sensor is coupled to the switching element or the bracket.
In the power tool of Japanese Laid-Open Patent Publication No. 2010-214520, the air current produced by the fan flows from the switching elements to the motor. This cools the switching element and the motor. However, the heat radiation member is dedicated to the switching elements, and thus does not radiate heat from the motor. Further, in the power tool of Japanese Laid-Open Patent Publication No. 60-207776, the switching element is coupled to the stator by the bracket. Thus, the heat of the motor hinders cooling of the switching elements.
In this manner, in the power tools of Japanese Laid-Open Patent Publication Nos. 2010-214520 and 60-207776, the cooling effect differs between the switching elements and the motor. Thus, when stopping the supply of power to the motor upon detection of overheating in the switching element, the temperature of the motor may be less than or equal to the tolerable limit temperature or may already be greater than the tolerable limit temperature.